A hydraulically actuated Jack And Pull (JAP) tubular expansion tool string comprises: an annular non-return valve arranged in the wall of the JAP string, which allows mud to flow from a surrounding annulus into the bore of the expansion tool string when the pressure in the bore of the tool string is lower than that in the annulus; a second non-return valve arranged in the JAP string below the annular valve which allows fluid to move down only; wherein during the cycling of the jack the annular non-return valve is continuously open and the second non-return valve cycles with the jack movement to inhibit curing of cement in the expansion tool string.